The invention relates to a process for the production of storage-stable epoxy prepregs and to composite components (mouldings) produced therefrom. The prepregs and components are produced by mixing acids and epoxides, where at least one of the two components comprises groups amenable to free-radical polymerisation. It is optionally possible to add, to the mixture, other epoxides and/or acids not amenable to free-radical polymerisation, and also free-radical initiators, e.g. photoinitiators. This mixture or solution is applied by known processes onto fibre material, e.g. carbon fibres, glass fibres or polymer fibres, and polymerised by radiation or plasma methods.
The polymerisation reaction, e.g. at room temperature or at up to 80° C., gives thermoplastics or thermoplastic prepregs which can then subsequently be subjected to a forming process. Elevated temperature can then be used to crosslink the acid-functionalised constituents with the epoxides already present in the system. It is thus possible to produce dimensionally stable thermosets and, respectively, crosslinked composite components.
Fibre-reinforced materials in the form of prepregs are used in many industrial applications because of their ease of handling and increased efficiency during processing, compared with the alternative wet-layup technology.
Industrial users of systems of this type demand not only lower cycle times and better storage stability values—at temperatures including room temperature—but also the possibility of cutting the prepregs to size without the problem of contamination of the cutting tools by the, often sticky, matrix material during automated cutting-to-size and layup of the individual prepreg layers.
Various moulding processes, e.g. the reaction transfer moulding (RTM) process, comprise the introduction of the reinforcing fibres into a mould, closing of the mould, introduction of the crosslinkable resin formulation into the mould, and subsequent crosslinking of the resin, typically by introduction of heat.
One of the limitations of a process of this type is that insertion of the reinforcing fibres into the mould is quite difficult. The individual layers of the woven fabric or laid scrim must be cut to size and adapted to the various geometries of moulds. This can be both time-consuming and complicated, particularly if the mouldings are also intended to comprise foam cores or other cores. Premouldable fibre reinforcement systems permitting easy handling and use of existing methods for forming processes would be advantageous.
Within the field of crosslinking matrix systems in addition to polyesters, vinyl esters and epoxy systems, there are a number of specialised resins. These include polyurethane resins; because these are tough, damage-tolerant and strong they are in particular used for the production of composite profiles by pultrusion processes. A disadvantage noted with such systems is that the isocyanates used may be toxic.
Prepregs and composites produced therefrom, based on epoxy systems, are described in WO 98/50211, EP 309 221, EP 297 674, WO 89/04335 and U.S. Pat. No. 4,377,657. WO 2006/043019 describes a method for the production of prepregs on the basis of epoxy-resin polyurethane powders. Prepregs based on pulverulent thermoplastics as matrix have also been disclosed. WO 99/64216 describes prepregs and composites, and also a method for production thereof by using emulsions with polymer particles sufficiently small to permit single-fibre coating. The viscosity of the polymers of the particles is at least 5000 centipoise, and these polymers are either thermoplastics or crosslinking polyurethane polymers.
EP 590 702 describes powder impregnation methods for the production of prepregs in which the powder is composed of a mixture of a thermoplastic and a reactive monomer and, respectively, prepolymers. WO 2005/091715 likewise describes the use of thermoplastics for the production of prepregs.
Prepregs with a matrix based on 2-component polyurethanes (2-C PUR) have been described. The 2-C PUR category comprises the traditional reactive polyurethane resin systems. In principle this is a system made of two separate components. While the critical constituent of one of the components is always a polyisocyanate, e.g. polymeric methylenediphenyl diisocyanates (MDI), the second component is a polyol or polyol mixture or an amino or amine-polyol mixture. The two parts are not mixed with one another until shortly prior to processing. This is then followed by chemical hardening through polyaddition, with formation of a network made of polyurethane and, respectively, polyurea. 2-component systems have a limited processing time (operating time, pot life) after the mixing of the two constituents, since the onset of reaction leads to gradual viscosity increase and finally to gelling of the system. Effective processability time may be determined by numerous variables: reactivity of the reactants, catalysis, concentration, solubility, moisture content, NCO/OH ratio and ambient temperature are the most important [see: Lackharze, [Coating Resins], Stoye/Freitag, Hauser-Verlag 1996, pp. 210/212]. The disadvantage of the prepregs based on 2-C PUR systems of this type is that only a short period of time is available for the processing of the prepreg to give a composite. The storage stability of prepregs of this type is therefore no more than a number of hours, rather than days.
Apart from the different underlying binder, moisture-curing coatings largely correspond to analogous 2C systems both in terms of their composition and in terms of their properties. In principle, the same solvents, pigments, fillers and auxiliaries are used. These systems are unlike 2C coatings in that for reasons of stability they cannot be exposed to any moisture at all before they are applied.
DE 102009001793 and DE 102009001806 describe a method for the production of storage-stable prepregs essentially composed of A) at least one fibrous substrate and B) at least one reactive pulverulent polyurethane composition as matrix material. The systems may also comprise poly(meth)acrylates as co-binder or polyol component. In DE 102010029355 compositions of this type are introduced into the fibre material by a direct melt impregnation process. In DE 102010030234 the method is pretreatment with solvents. These systems have the disadvantage of high melt viscosity and, respectively, the use of solvents which at some stage require removal, and can also have attendant toxicological disadvantages.
In view of the disadvantages and problems associated with the above described conventional systems, it was an object of the present invention to provide novel prepreg technology that can lead to a simpler process for the production of prepreg systems which give no handling problems.
A particular object of the present invention was to provide an accelerated process which can produce prepregs and which, in comparison with known technology, may give a markedly increased storage stability period and/or processing time, for example operating time, and pot life.